I think he's the hardest Marvel character to make a solo movie around.
Banner is too much of a loner, and there's really only so much you can do with him and Betsy Ross moping about his condition while Thunderbolt angrily plans to mobilize against him. The version of the Hulk they want to use is just very limiting with plot. And you're only going to have 2 kinds of major action scenes: The military trying to blow him up or him trading punches with another gamma-irradiated monster, neither of which make very compelling action sequences long-term. Not to mention that the bulk of his potential allies are all gamma monsters as well.
It's hard to build strong pathos around that, at least until they adopt the more merged Hulk who is smarter.
I think there are two possible stories to tell with the Hulk that touch upon what has been mentioned in this thread and your specific complaint. I think either doing the Planet Hulk movie or doing a Hulk team-up movie with one of the other Avengers as his buddy would be the best way to go. Planet Hulk would work because it taps into another micro-genre movie, the 'swords and sandals' Spartacus/Gladiator type of movie, that Marvel suggests they're doing moving forward other than doing 'just comic book movies'. The other would be to do a combination of a buddy movie and a road trip movie (think Planes, Trains, and Automobiles with superheroes). Hawkeye would probably be the best bet, because he's more of the quiet, brooding type in the movies whereas Hulk is a house of fire. So play them up as an odd couple who have to reluctantly team up for some reason. The Earth's Mightiest Heroes cartoon a couple of years back made this relationship work in such a way and it was really fun to watch.
But in either movie, what has to be done, which touches upon what you said, is that the pathos has to be built up. I think the best way to do that, and again the EMH cartoon did this really well, is you play up the Hulk as the unintentional hero in spite of himself. Some of Whedon's recent comments about the Hulk in Age of Ultron hint at this: he's really a werewolf in superhero's clothing rather than a superhero in a werewolf's clothing. I think this dynamic is how someone like Ruffalo, as a de-Hulked Banner or psychic entity independent of the Hulk, can do on his own. Add in that Ruffalo could also do a ton of mo-cap work when playing the Hulk, then you have both sides of the character's personality come into it. I go back to the EMH cartoon because it did this well: have the Hulk make a choice to be the superhero. Even in the first Avengers, he's not so much a hero of his own volition, other than he directs his rage against bad guys (when before he attacked the good guys just as indiscriminately). There is always the fear that the Hulk will lose control (Hulkbuster fight perhaps?). On one hand, that's unavoidable. On the other hand, even the good guys treat the Hulk not as a fellow hero, but the heavy weapon they throw at a problem.
So why not have Banner/Hulk deal with that? In a Planet Hulk storyline, Hulk is the reluctant hero of a slave/gladiator rebellion even though his motivations are simply to get back to Earth. But along the way, he discovers the plight of the slave/gladiator class and decides to step up and be the hero the slaves need to become free, a hero who leads the way, instead of being just the "HULK SMASH!" guy. In a Hulk buddy road movie, have Hulk (somehow) verbalize other people's perceptions of him as a monster, and Hawkeye could be the guy to first see the Hulk as an instrument to then become his friend and equal, or have him simply be a sounding board to the Hulk's feelings, or have him be a non-Hulk powered pain in the ass who is also viewed as a troublemaker (which is what the EMH cartoon basically did).
The only thing I would add is that if Marvel decides to do a Planet Hulk movie, the Hulk can't be sent into space by the heroes, because then they come off like assholes and that will just needlessly muddy the waters.. Maybe have Thanos punch the Hulk out of the galaxy in one of the Avengers films, and then come Phase 4 you show the Hulk lands on Sakar. Hell, if Marvel can get the Fantastic Four rights back before then, set up the Silver Surfer in the movie to then pay him off with a full-blown Fantastic Four movie as a part of Phase 4.
A road trip movie of sorts, where he hsa someone to play off of, could be really fun.
The big problem with doing a movie with a CGI Hulk is you need a substantial budget otherwise your title character is only going to be onscreen for maybe 20 minutes. They would have been better off with a human actor made to look bigger through camera tricks and other special effects. That allows you to use him more and to build a better connection with the audience.